Prospects for $\Tau $ Lepton Physics at Belle II

$Prospects for $\Tau $ Lepton Physics at Belle II$

Flavor Physics and CP Violation Conference, Victoria BC, 2019 1

Prospects for τ Lepton Physics at Belle II

D. Rodr´ıguez Pérez on behalf of Belle II Collaboration Facultad de Informática Culiacán,Universidad Autónomade Sinaloa, Cd. Universitaria, CP 80013, Sinaloa, México.

The Belle II experiment is a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e+e− collider. The design luminosity of the machine is 8×1035 cm−2s−1 and the Belle II experiment aims to record 50 ab−1 of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run and main operation of SuperKEKB has started in March 2019. Belle II has a broad τ physics program, in particular in searches for lepton flavor and lepton number violations (LFV and LNV), benefiting from the large cross section of the pairwise τ lepton production in e+e− collisions. We expect that after 5 years of data taking, Belle II will be able to reduce the upper limits on LFV and LNV τ decays by an order of magnitude. Any experimental observation of LFV or LNV in τ decays constitutes an unambiguous sign of physics beyond the Standard Model, offering the opportunity to probe the underlying New Physics. In this talk we will review the τ lepton physics program of Belle II.

I. INTRODUCTION Belle II keeps the design of the previous detector Belle, with major upgrades in each of their subsystems. The The SuperKEKB [1] accelerator is upgraded from main modifications are: KEKB and its target luminosity is 8 × 1035 cm−2s−1, 40 times higher than KEKB. SuperKEKB collides • The vertex detector, which contains two layers electrons and positrons at the Υ(4S) resonance en- of DEPFET pixel (PXD) and four layers of sil- ergy, producing a large amount of B meson pairs, a icon strips (SVD), improving the resolution re- B-factory. However, the cross section of the process spect to Belle. + − + − e e → τ τ at the Υ(4S) resonance energy is of • The central drift chamber (CDC) has a larger the same order as the production of a B pair, then, volume with smaller drift cells. SuperKEKB is also a τ lepton factory. The Belle II [3] detector has been upgraded from • A completely new particle identification system, Belle detector to cope with higher luminosity and using aerogel ring-imaging Cherenkov detectors. higher expected beam backgrounds. It is a second generation B-factory and offers unique capabilities to • Faster electronics in general. study B and τ decays with neutrinos in the final A complete description of the Belle II detector can states, with respect to experiments at hadron collid- be read at reference [3]. ers. From April to July of 2018, Belle II performed the The Belle II program is based on the success of the Phase II of commissioning. Detector recorded 500 BaBar at SLAC and Belle at KEK, the first generation pb−1 of data at Υ(4S) energy with the BEAST II of B-factories, which discovered and established vio- detector installed, instead of the vertex detector. The lation of charge-parity symmetry (CPV) in B meson BEAST II contains slice of the vertex detector and dynamics, furthermore they performed precision mea- radiation monitors used in beam background studies surements of τ properties, such as the mass, lifetime [4]. Later in 2018, BEAST II was replaced by the ver- arXiv:1906.08950v1 [hep-ex] 21 Jun 2019 and branching fractions of leptonic and semileptonic tex detector and in March 2019, Belle II was started decays. Additionally, they imposed limits in electric Phase III of data with all the subsystems installed, dipole moment, lepton flavor violation (LFV) and lep- expecting a full dataset of 50 ab−1 by the end of the ton number violation (LNV) decays [2]. data taking, in 2025. Now, Belle II has an ambitions program in τ physics, aiming to move down the upper limit of the rate of LFV and LNV τ decays by two order of mag- III. FIRST RESULTS OF τ LEPTON nitude. PHYSICS AT BELLE II

A. Reconstruction of τ Pair Production II. THE BELLE II EXPERIMENT The reconstruction of the tau pair production The Belle II experiment is a detector coupled to the e+e− → τ +τ − is performed searching 3-1 prong SuperKEKB accelerator, located in Tsukuba, Japan. events in a data sample of 291 pb−1. Events in

PSN fpcp TueB1700 2 Flavor Physics and CP Violation Conference, Victoria BC, 2019

data are required to ﬁre the CDC trigger. Only four charged tracks per event are accepted, with zero net charge and splitting the decay products into two op- posite hemispheres by a plane perpendicular to the thrust axis ~n¯thr, deﬁned such that

P cm i |~pi · nˆthr| Vthr = P cm (1) |~pi | cm is maximized, with ~pi being the momentum in the center-of-mass system (CMS) of each charged particle and photon. Signal side hemisphere is deﬁned as the one containing a 3-prong decay, while the tag FIG. 2: Event desplay of the Belle II detector showing a + − side should contain the 1-prong decay. A pion mass 3-1 prong event, likely a e e → (τsig → 3πντ )(τtag → hypotesis is used for all charged tracks, looking for µντ ν¯µ) candidate reconstructed in Phase II data [5]. τ → 3πν events in the signal side. After further selection criteria are applied, 9800 events remain as τ pair candidates. Figure 1 shows is obtained for each τ → 3πν candidate. Here, Ebeam the invariant mass distribution of the three charged is the energy of one of the beams in CMS and M3π, pions coming from τ → 3πν candidates, with Monte E3π and P3π represent the invariant mass, the en- Carlo (MC) simulated events superimposed. Figure 2 ergy and the momentum of the hadronic system of presents the event display of the Belle II detector with the three pions in CMS, respectively. one of the candidates who pass the cuts. An empirical probability density function (p.d.f.) is used to estimate the τ lepton mass. The edge p.d.f. used is described by

∗ F (Mmin; a, b, c, m ) = (a ∗ Mmin + b) × ∗ arctan[(m − Mmin)/c] +

P1(Mmin), (3)

in which a, b and c are real values and the paramenter m∗ is an estimator of the τ lepton mass. A fit of the p.d.f. (3) in the pseudomass region from 1.70 to 1.85 GeV/c2, yields a mass measurement 2 FIG. 1: Invariant mass distribution of the three pions com- of mτ = (1776.4 ± 4.8(stat)) MeV/c . Figure 3 shows ing from τ → 3πν candidates reconstructed in Phase II the pseudomass distribution of the τ → 3πν candi- data [5]. Events in data are required to fire the CDC dates, with the p.d.f fitted superimposed. The result trigger. MC is rescaled to a luminosity of 291 pb−1 and is in good agreement with the measurements from the reweighted according to the trigger efficiency measured in previous experiments [7]. data. The error band on the total MC includes the MC statistical uncertainty, the luminosity uncertainty and the uncertainty associated with the trigger efficiency reweight- ing. IV. PROSPECTS FOR τ LEPTON PHYSICS

The goal of Belle II experiment is to focus on preci- B. τ Lepton Mass Measurement sion measurements and extrapolation of new physics (NP) in rare physics processes, and with 45 billions A first τ lepton mass measurement at Belle II is per- of e+e− → τ +τ − events expected at the end of the formed following the method develop by the ARGUS data taking, the study of τ physics will be possible. collaboration [6]. The pseudomass Mmin, defined by Prospect for τ lepton physics at Belle II are briefly discribed . Further details and a more complete de- q scription may be found in the Belle II Physics Book 2 Mmin = M3π + 2(Ebeam − E3π)(E3π − P3π), (2) [8].

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FIG. 4: Current 90% C.L. upper limits for the branching fraction of LFV τ decays. Limits imposed by CLEO, BaBar and LHCb are showed. Additionally, prospects for limits to be imposed by Belle II are indicated with red circles, assuming an integrated luminosity of 50 ab−1 [8].

B. CP Violation in τ Decays

FIG. 3: Distribution of pseudomass Mmin = p 2 − It is also expected that Belle II will measure CP M3π + 2(Ebeam − E3π)(E3π − P3π) of τsig → 3πν candidates reconstructed in Phase II data. The asymmetries in τ decays at a level that bounds many blue line is the result of an unbinned max- models of NP in a complementary way to the LFV 0 imum likelihood fit, using an edge function searches. For example, CPV in τ → KSπν. This (a ∗ M + b) · arctan[(m∗ − M )/c] + P (M ), 0 min min 1 min decay of the τ lepton to final states containing a KS in which m∗ estimates the τ lepton mass. A mass of 2 meson will have a nonzero decay-rate asymmetry Aτ , mτ = (1776.4 ± 4.8(stat)) MeV/c is measured [5]. defined by

+ + 0 − − 0 Γ(τ → π KSν¯τ ) − Γ(τ → π KSντ ) Aτ = + + 0 − − 0 , (4) Γ(τ → π KSν¯τ ) + Γ(τ → π KSν¯τ ) A. Lepton Flavor Violation in τ Decays due to CP violation in the kaon sector. The SM prediction [14, 15] yields

The discovery of neutrino oscillations has demon- ASM = (3.6 ± 0.1) × 10−3. (5) strated that lepton flavor number are not conserved in τ the neutrino sector and claims for an extended Stan- On the experimental side, BaBar is the only exper- dard Model (SM) with massive neutrinos. But, if the iment that has measured Aτ [16], getting SM is extended to include neutrino masses only, the branching ratio of lepton flavor violation (LFV) pro- BaBar −3 cesses is too small, ∼ 10−54, to be observed [9, 10]. Aτ = (−3.6 ± 2.3 ± 1.1) × 10 , (6) Then, Belle II will be very competitive to explored which is 2.8σ away from the SM prediction (5). An NP in this field for its relatively low background [11]. improved measurement of Aτ is a priority at Belle II. CP violation could alse arise from a charged scalar The golden channels for studying charged LFV are boson exchange. It can be detected as a difference τ → 3µ and τ → µγ. The first one is a purely leptonic in the decay angular distributions. Belle searched state and the background is suppressed; the second for CP violation in angular observables of the decay 0 one has the largest LFV branching fraction in models τ → KSπν [17], in which almost all contributions to where the decay is induced by one-loop diagrams with systematic uncertainty depend on the control sample heavy particles [12, 13]. statistics. So, it is expected that the uncertainties√ at Belle II will be improved by a factor of 70, given the Figure 4 shows the prospects for upper limits to integrated luminosity projected. be imposed in τ LFV decays according to sensitivity studies described at [8] and, for comparison, the limits imposed for previous experiments. With the full V. CONCLUSIONS dataset expected for the Belle II experiment, 50 ab−1, the upper limit for the branching fraction of LFV de- Belle II has recorded successfully ∼ 500 pb−1 cays τ will be reduced by two orders of magnitude. of data during the first collisions performed at Su-

PSN fpcp TueB1700 4 Flavor Physics and CP Violation Conference, Victoria BC, 2019

perKEKB during Phase II. The data has been used not consider. Additionally, a good agreement between mostly for beam background and detector perfor- data and simulation is observed. mance studies, showing a healthy operation of all The τ lepton physics program at Belle II will take the subsystems. This year, full physics program has advantage of the largely integrated luminosity ex- started and by the end of the experiment, in 2025, pected, allowing the study of several topics. Limits Belle II is expected to collect 50 ab−1 of data. in branching ratio of LFV decays, CP violation asym- The preliminary result on τ lepton mass measure- metry parameters will be improved by two orders of ment obtained from Phase II data, mτ = (1776.4±4.8) magnitude, but a careful analysis of systematic uncer- MeV, is in good agreement with the measurements tainties is required. reported by previous experiments and the average τ mass value by the PDG. Systematic uncertainties were

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